The structure and operation of hard disk drives is generally known. Hard disk drives include, generally, a case, a hard disk having magnetically alterable properties, and a read/write mechanism including Read/Write (RW) heads operable to write data to the hard disk by locally alerting the magnetic properties of the hard disk and to read data from the hard disk by reading local magnetic properties of the hard disk. The hard disk may include multiple platters, each platter being a planar disk.
All information stored on the hard disk is recorded in tracks, which are concentric circles organized on the surface of the platters. FIG. 1 depicts a pattern of radially-spaced concentric data tracks 12 within a disk 10. Data stored on the disks may be accessed by moving RW heads radially as driven by a head actuator to the radial location of the track containing the data. The track-based organization of data on the hard disk(s) allows for easy access to any part of the disk, which is why hard disk drives are called “random access” storage devices.
Since each track typically holds many thousands of bytes of data, the tracks are further divided into smaller units called sectors. This reduces the amount of space wasted by small files. Each sector holds 512 bytes of user data, plus as many as a few dozen additional bytes used for internal drive control and for error detection and correction.
Typically, these tracks and sectors are created during the low level formatting of the disk. This low level formatting process creates the physical structures (tracks, sectors, control information) on the disk. Normally, this step begins with the hard disk platters containing no information. Newer disks use many complex internal structures, including zoned bit recording to put more sectors on the outer tracks than the inner ones, and embedded servo data to control the head actuator. Newer disks also transparently map out bad sectors. Due to this complexity, all modern hard disks are low-level formatted at the factory for the life of the drive.
This low level formatting is usually performed using external servo writers that write the physical structures to disk 10 during manufacturing. Accurate positioning of the physical structures is achieved within the external servo writer by accurately controlling the RW head position within the external servo writer. External servo writers, because of the high degree of positioning accuracy required, have become an expensive processing bottleneck during the hard disk drive manufacturing.
Self servo writing (SSW) attempts to overcome this expensive and time-consuming bottleneck. External servo writers write a first magnetic reference pattern (servo pattern) on the surface of the disk. The disk may then be assembled into a hard disk drive, where the LLF is initially performed within the hard disk drive. In SSW, the LLF is completed within the hard disk drive without using additional external hardware. The LLF uses the servo pattern written by the external servo writer to create the physical structures on the disk.
During most SSW processes, the position of the RW heads may drift from their targeted position. This drift may introduce a position error following completion of the SSW process. Further, since the position of each subsequent radial track may depend on the position of the previously written track, this position error can accumulate during the SSW process. Therefore, the accumulation of positioning errors should be addressed such that the position errors accumulated during the SSW process may be reduced or eliminated.